Remove 18 HAVE_ macros that were unused from config.h.in.

Goodbye to:
HAVE_DIRENT_H HAVE_ERRNO_H HAVE_FLOAT_H HAVE_HSTRERROR HAVE_LIMITS_H
HAVE_MCHECK_H HAVE_MTRACE HAVE_NETDB_H HAVE_PTHREAD_H
HAVE_SECURITY_PAM_APPL_H HAVE_SOCKET_H HAVE_STRSIGNAL HAVE_SYSINT_H
HAVE_SYS_IPC_H HAVE_SYS_SEM_H HAVE_SYS_SHM_H HAVE_VALUES_H